Objection 1. It would seem that venial sin is unfittingly condivided with mortal sin. For Augustine says (Contra Faust. xxii, 27): "Sin is a word, deed or desire contrary to the eternal law." But the fact of being against the eternal law makes a sin to be mortal. Consequently every sin is mortal. Therefore venial sin is not condivided with mortal sin.
Objection 2. Further, the Apostle says (1 Corinthians 10:31): "Whether you eat or drink, or whatever else you do; do all to the glory of God." Now whoever sins breaks this commandment, because sin is not done for God's glory. Consequently, since to break a commandment is to commit a mortal sin, it seems that whoever sins, sins mortally.
Objection 3. Further, whoever cleaves to a thing by love, cleaves either as enjoying it, or as using it, as Augustine states (De Doctr. Christ. i, 3,4). But no person, in sinning, cleaves to a mutable good as using it: because he does not refer it to that good which gives us happiness, which, properly speaking, is to use, according to Augustine (De Doctr. Christ. i, 3,4). Therefore whoever sins enjoys a mutable good. Now "to enjoy what we should use is human perverseness," as Augustine again says (Qq. lxxxiii, qu. 30). Therefore, since "perverseness" [The Latin 'pervertere' means to overthrow, to destroy, hence 'perversion' of God's law is a mortal sin.] denotes a mortal sin, it seems that whoever sins, sins mortally.
Objection 4. Further, whoever approaches one term, from that very fact turns away from the opposite. Now whoever sins, approaches a mutable good, and, consequently turns away from the immutable good, so that he sins mortally. Therefore venial sin is unfittingly condivided with mortal sin.
On the contrary, Augustine says (Tract. xli in Joan.), that "a crime is one that merits damnation, and a venial sin, one that does not." But a crime denotes a mortal sin. Therefore venial sin is fittingly condivided with mortal sin.
I answer that, Certain terms do not appear to be mutually opposed, if taken in their proper sense, whereas they are opposed if taken metaphorically: thus "to smile" is not opposed to "being dry"; but if we speak of the smiling meadows when they are decked with flowers and fresh with green hues this is opposed to drought. In like manner if mortal be taken literally as referring to the death of the body, it does not imply opposition to venial, nor belong to the same genus. But if mortal be taken metaphorically, as applied to sin, it is opposed to that which is venial.
For sin, being a sickness of the soul, as stated above (71, 1, ad 3; 72, 5; 74, 9, ad 2), is said to be mortal by comparison with a disease, which is said to be mortal, through causing an irreparable defect consisting in the corruption of a principle, as stated above (Question 72, Article 5). Now the principle of the spiritual life, which is a life in accord with virtue, is the order to the last end, as stated above (72, 5; 87, 3): and if this order be corrupted, it cannot be repaired by any intrinsic principle, but by the power of God alone, as stated above (Question 87, Article 3), because disorders in things referred to the end, are repaired through the end, even as an error about conclusions can be repaired through the truth of the principles. Hence the defect of order to the last end cannot be repaired through something else as a higher principle, as neither can an error about principles. Wherefore such sins are called mortal, as being irreparable. On the other hand, sins which imply a disorder in things referred to the end, the order to the end itself being preserved, are reparable. These sins are called venial: because a sin receives its acquittal [veniam] when the debt of punishment is taken away, and this ceases when the sin ceases, as explained above (Question 87, Article 6).
Accordingly, mortal and venial are mutually opposed as reparable and irreparable: and I say this with reference to the intrinsic principle, but not to the Divine power, which can repair all diseases, whether of the body or of the soul. Therefore venial sin is fittingly condivided with mortal sin.
Reply to Objection 1. The division of sin into venial and mortal is not a division of a genus into its species which have an equal share of the generic nature: but it is the division of an analogous term into its parts, of which it is predicated, of the one first, and of the other afterwards. Consequently the perfect notion of sin, which Augustine gives, applies to mortal sin. On the other hand, venial sin is called a sin, in reference to an imperfect notion of sin, and in relation to mortal sin: even as an accident is called a being, in relation to substance, in reference to the imperfect notion of being. For it is not "against" the law, since he who sins venially neither does what the law forbids, nor omits what the law prescribes to be done; but he acts "beside" the law, through not observing the mode of reason, which the law intends.
Reply to Objection 2. This precept of the Apostle is affirmative, and so it does not bind for all times. Consequently everyone who does not actually refer all his actions to the glory of God, does not therefore act against this precept. In order, therefore, to avoid mortal sin each time that one fails actually to refer an action to God's glory, it is enough to refer oneself and all that one has to God habitually. Now venial sin excludes only actual reference of the human act to God's glory, and not habitual reference: because it does not exclude charity, which refers man to God habitually. Therefore it does not follow that he who sins venially, sins mortally.
Reply to Objection 3. He that sins venially, cleaves to temporal good, not as enjoying it, because he does not fix his end in it, but as using it, by referring it to God, not actually but habitually.
Reply to Objection 4. Mutable good is not considered to be a term in contraposition to the immutable good, unless one's end is fixed therein: because what is referred to the end has not the character of finality.
Source: Summa Theologica (New Advent)